Momentum is a very useful concept owing largely to the fact that as we shall see shortly, it is conserved. Like conservation of energy, this allows you to solve a lot of problems that would be very difficult to solve any other way. One important application of this conservation law, is to the study of collisions. Collisions occur when two objects, or people, bump into each other. Besides exchanging pleasantries such as ``Excuse me, I didn't mean to break your nose", they exchange some of their energy and momentum. We will see how to use this to find out the trajectories of the objects after the collsion. But first we need to define momentum.